The present invention concerns a device for burying submarine cables or re-burying cables which are laid out on the sea and river beds, and a method for guiding the same.
The cables for communication, electric power etc. in recent years are more and more installed for longer distances and increasing number of the cables have been laid out upon sea or river beds. Since this type of cable is required to offer a higher reliability, various methods have been devised for installation thereof; for instance, there has been widely used an installation method to bury the cable under the sea or river beds in a fairly shallow water area in order to protect the cable from damage which might be caused by fishing gear or ship anchors. For maintaining high reliability of the cable system it is of utmost importance to have sufficient technology required to monitor the cable installation works, to repair it promptly at the time of failure and/or to give proper protection after such repair (such as re-burying works for the cables which have been excavated out of the bed for repair). Such technology has currently been the object of research and development in various countries.
In order to achieve proper works for monitoring, repair, protection, re-burying, etc. for the cables, it is required to have the technology to detect the position of the cable which has been buried under the sea bed, and to have cable tracking technology to move working machines accurately along the buried cable. This requires an extremely sophisticated technology of detecting the cable position especially when the cable to be detected has been buried or installed underneath the sea bed in the open sea of a deeper water depth.
The monitoring method with an underwater TV camera mounted on a working machine has generally been used in prior art for detection or tracking of the submarine cables. However, that method is not necessarily satisfactory since it cannot be applied to buried cables and further since its detection performance is greatly limited in the water of high turbidity. Another method for detection and tracking has been proposed to be effective under any conditions by supplying a signal current to the cable and by detecting the magnetic field which is generated by the signal current. The method using direct current (DC) as the signal current, however, is not quite effective because of the difficulty to distinguish between the DC magnetic field generated from the cable and the geomagnetic field. Therefore, the development of a method using AC has become urgently needed.
The submarine cable communication system using coaxial cables has drawbacks in that the desirable frequency which will not affect the frequency band to be used for communication is limited to a few hundred Hz and that if such low frequency alternating current (AC) current is supplied from a terminal, the AC current becomes attenuated by the internal power separating filters everytime it passes through a repeater/amplifier (referred to as repeater hereinafter) inserted on the path to become almost zero after passing a few repeaters. Therefore, a method to supply an AC signal current to the cable from a place other than the terminal is strongly demanded.
The present invention aims to obviate such defects of the conventional method, and at the same time, to offer a device for burying the submarine cable under the water bed by easily and accurately detecting the cable position and a method for guiding the same by utilizing the characteristics unique to submarine cables, by supplying an AC only to the portion of the cable between two arbitrary repeaters through a so-called transformer-coupling system to generate an AC magnetic field, and by detecting the magnetic field with a magnetic field detector.
The method and the device according to the present invention will now be described in more detail referring to the embodiments indicated in attached drawings.